1. Field of Endeavor
The present invention relates to the field of turbomachines. It relates to a rotor blade for fastening on the rotor of a turbomachine, to a method for producing a rotor blade, and also to a compressor with such a rotor blade.
2. Brief Description of the Related Art
The rotor blades of a compressor are part of an axial compressor system which moves and compresses large quantities of air which are required for the correct and reliable operation of a gas turbine system. The rotor blades are mounted on the outer periphery of the rotor of the compressor and are subjected to a large number of mechanical loads which especially also depend upon the type of blade fastening.
In order to reliably avoid the detachment of a rotor blade from the rotor, which is catastrophic for the system, various systems have been developed and proposed in the past for fastening the rotor blades on the rotor. One of these systems, to which the present invention relates, is the fastening system with a T-shaped blade root and spacers, as is reproduced in detail in a simplified form in FIGS. 1 and 2, and in FIG. 6, and which has been known for a long time for example from publication DE-PS-318 662.
In the case of this system, spacers 12, 18 are inserted one behind the other in the circumferential direction in a slot 11 which extends around the rotor axis (A2 in FIG. 5) of the rotor 10 and are retained in the slot 11 by retaining surfaces 20 which abut against undercuts 24. A rotor blade 13 or 27 is arranged in each case between two adjacent spacers 12, 18 and includes a blade airfoil 14, and by a T-shaped blade root 15 or 25 (see FIG. 6) which adjoins the blade airfoil 14 at the bottom and abuts against the side surfaces of the adjacent spacers 12, 18, and by shoulders 16, 16′ or 26, 26′ which project in the circumferential direction and fit under the adjacent spacers 12, 18. The spacers 12, 18 and the rotor blades 13 or 27 in this case are arranged at an angle to the rotor axis A2, so that the blade axis (A1 in FIG. 5) with the rotor axis A2 includes an angle of, for example, 25° (see FIG. 5).
The T-shaped blade roots 25 of the compressor rotor blades 27 have previously been formed (forged) by upset forging and so have obtained a grain structure which determines the strength, as is indicated in FIG. 6 by broken lines. In recent times, dependent upon new requirements with regard to costs, tools, and logistics, the changeover is increasingly being made to no longer forging the blade roots but producing the blade roots by milling (grain structure in FIG. 4). In order to achieve the same or an improved strength at the critical transitions to the shoulders 16, 16′ or 26, 26′ in the case of the milled blade roots, as in the case of the forged blade roots, a larger radius has to be provided at the transitions. In the case of the forged blade roots 25, the forged radius 29 lies approximately in the region of between 0.5 and 1.0 mm (FIG. 6). On account of the notch factor, milled blade roots require at the transitions a radius which is about 1.5 to 2 times larger than the forged radius 29.
The previous forging method for the blade roots 25 had further consequences: as a result of the upset forging, bulges 31 occur on the shank above the shoulders 26, 26′ (in FIG. 6 this is indicated by the dotted lines) which lie within the range of 0.3 to 0.5 mm. So that the rotor blades installed with the forged blade roots 25, despite the side bulges 31, nevertheless abut securely and immovably against the adjacent spacers 12, 18, they are provided with a long chamfer 17, 19 on the side surfaces in the lower section, which creates the space for the bulges 31 (see also FIG. 3).
Although, depending upon the blade size, a 0.3 to 0.5 mm wide gap already exists between the adjacent spacers 12, 18 and the T-shaped blade root 25, an increase of the radius in the corners of the shoulders 26, 26′ by the factor 1.5 to 2, as is required for milled blade roots, would cause an undesirable and hazardous collision exactly at the place where the relief of mechanical stresses actually should be the aim.
It is possible, on the other hand, to relieve mechanical stresses in the regions of machine parts in which, on the one hand, the risk of cracks is great, and on the other hand the space for the applying of larger radii, however, is limited, by providing suitably dimensioned and positioned relief grooves, according to ISO standard.